The drafting apparatus is indispensable for a spinning frame. The spinning speed in the spinning frame differs depending on whether the spinning frame is a ring spinning frame or a pneumatic spinning frame. Ordinarily, in the ring spinning process, the spinning speed is 13 to 15 m/min in case of a yarn of a British count of Ne 45, and in the pneumatic spinning process, it is said that a spinning speed of 180 to 200 m/min is attainable in case of the above yarn. In each of these two spinning processes, it is not too much to say that the physical properties, that is, quality, of spun yarns depend mainly on the capacity of the drafting apparatus disposed before the spinning step. In the case of the pneumatic spinning process, since the spinning operation can be performed at a high speed, it is necessary to use a high-draft apparatus adaptable to high-speed spinning or a stepwide drafting apparatus including the roving and spinning steps. Accordingly, influences of the drafting apparatus are very significant.
In this drafting process, when the fed sliver is bent or flattened, uneven drafting takes place regardless of the functional condition of drafting system and, therefore, not only imperfections such as slubs or neps are generated on the yarn but also yarn breakage occurs more frequently, resulting in decrease of operation efficiency of the spinning frame and degradation of the yarn quality. Since the sliver that has undergone drawing process is generally coiled and contained in a cylindrical can, it is liable to be deformed very flat in cross-section and is, in some cases, shaped into a ribbon-like one upon receiving partial twisting when taken out from the can. When such deformed sliver as above is fed between back rollers of the spinning frame, a concurrent action of sliver-flattening by mutual compression of rollers and break draft between the back and middle rollers greatly expands the fiber bundle running between these two roller pairs and, in addition, produces variation in width every time of expanding, which leads to uneven draft.
In the drafting apparatus, a sliver is consecutively drafted by passing it through between pairs of back rollers, middle rollers and front rollers differing in the peripheral speed. Since the draft ratio between the middle rollers and the front roller is several times to scores of times as high as the draft ratio between the back rollers and the middle rollers, aprons are arranged on the middle roller as means for controlling occurrence of uneven drafting.
The aprons attached to the middle rollers are effective for controlling occurrence of uneven drafting and this effect is especially prominent in the high-draft apparatus. However, since a fiber bundle being drafted is passed through between the upper and lower aprons pressed to each other, the fiber bundle-passing surfaces of the aprons are worn by friction, and with advance of wearing of the aprons, the quality of the spun yarn is adversely influenced by wearing of the aprons. Especially in the high-draft apparatus where high-speed spinning is possible, wearing of the aprons is advanced very promptly, and from experiments made by us, it has been confirmed that in case of a fully operation, adverse influences on the physical properties of spun yarns are observed within about 3 to about 4 days in case of aprons formed of some materials and in such case, the worn aprons should be exchanged with new aprons.
The factor having influences on the physical properties of spun yarns is not limited to wearing of the aprons, but they are influenced by the surface roughness of the aprons and other spinning conditions. However, it is construed that the influences of wearing of the aprons are significant.
Exchange of aprons requires the troublesome operation of taking out the shafts of the middle rollers, and these shafts are integrally connected to those of other units, for example, 10 to 15 units. Accordingly, a series of the related units should be stopped for exchanging aprons in one unit, with the result that the productivity is reduced. Consequently, the conventional technique involves problems of early appearance of influences of wearing of aprons on the quality of spun yarns and also of reduction of the productivity of necessity of exchange aprons.